Review Reports

Round 1

Reviewer 1 Report

The chemical formula BaSrAl2O4 used in the title, abstract, keywords and along the manuscript is confusing. Is it a solid solution or a composition with the certain ratio of Ba and Sr or a mixture of SrAl2O4 and BaAl2O4? I think it is important to discuss this matter.

There is a lot of literature on Eu2+ doped BaAl2O4 and SrAl2O4. Please discuss this literature in introduction and compare you results with this literature data.

Line 70. Please explain how you chose the ratio of the components.

Line 74. Please explain the role of urea in the synthesis of the material.

Line 124. “The impurity phase is inferred to be BaO, and the ratio is calculated to be about 0.4%.” Please explain how you estimamed the composition of your material.

Line 130. “Eu2+ ions have been successfully incorporated in BSAO”. Please explain the reasons for this conclusion.

Line 100. Please correct the sentence, which does not look like a sentence. “Visible light optical fiber spectrometer (FX2000, Shanghai Fuxiang) to measure the X-ray fluorescence spectra of different samples by X-ray excited.”

Line 101. Please correct the sentence, which does not look like a sentence. “UV-3600 UV-Vis Near Infrared Spectrophotometer (Shimadzu Corporation, Japan).”

Line 106. Please correct typo in the formula Ba0.05Sr0.05Al2O4.

Line 131. Please correct typo in the phrase “Generally, It is reasonable…”

Fig. 1(a). Please give your comments on the appearance of two peaks at 2theta~20 degrees and on variation of their intensities with Eu2+ content. Please be aware that in the standard PDF card there is only one peak.

Fig. 1(c). Please give your comments on the evolution of two peaks at 2theta~20 degrees with the reaction temperature.

The unit cell parameters of BaAl2O4 are a = b= 10.449, c = 8.793 Å [M. Malkamäki, A.J.J. Bos, P. Dorenbos, M. Lastusaari, L.C.V. Rodrigues, H.C. Swart, J. Hölsä, Persistent luminescence excitation spectroscopy of BaAl2O 4:Eu 2+ ,Dy3+ , Phys. B Condens. Matter (2020), https://doi.org/10.1016/j.physb.2019.411947.]

In this paper, it is written that the unit cell parameters are  a = b = 8.926 Å, c = 8.4985 Å for pure Hexagonal SrAl2O4 (PDF#97-015-3164).

“The calculated cell parameters of a = b = 8.972 Å, c = 8.606 Å for BSAO” are not “in good agreement with the standard data of a = b = 8.926 Å, c = 8.4985 Å for pure Hexagonal SrAl2O4 (PDF#97-015-3164)”, line 127, but are between the parameters for BaAl2O4 and SrAl2O4. Please make correction.

Fig. 2. Please present the chemical composition in atom %.

Fig. 2 caption. Please indicate the reaction temperature of the sample which EDS data is presented in Fig 2(f).

Line 165. Please correct the description of the Fig. 3a, “The absorption spectrum shows a plateau for wavelengths above 460 nm and a steep decrease with shoulders from 230 to 270 nm.”, which does not match the figure.

Equation (3), Figure 5 caption. Please explain the meaning of “x”.

Please correct Figure 6 caption. It shoud be written “…under the excitation of (a) 303 nm, (b) 365 nm;”.

Lines 211-214. I think, it is a contradiction between the sentences “The reaction temperature is a highly essential aspect that directly impacts the structure and luminescent properties of the sample.“ and “When the reaction temperatures were different, there was no significant difference in the spectral features at the same wavelength excitation.”

Fig. 6(d). Please change the color of the symbol, which shows the chromaticity coordinates of 0.176 and 0.365. The present color is hardly noticeable.

Line 226. “Moreover, the luminescence decay curves of the BSAO: xEu2+ (x=0.01, 0.02, 0.03, 0.04, 226 0.06) phosphor (lex = 303 nm, lem = 442 nm; lex = 365 nm, lem = 501 nm) are shown in Figure 7.” This sentence is in a contradictory with Figure 7 and its capture. Please make correction. Please check tau ave value presented in Figure 7(a).

Line 230. Please correct typo in the words “decay time”.

Line 235. Please correct the typo. It should be written “Furthermore, photoluminescence…”

Line 234. “According to the formula, the average lifetime of BSAO: 0.02Eu2+ is 355.33ns and 111.98 ns” Please provide the error analysis and make correction of these values accordingly.

Line 246. “QY could reach up to 10.49% and 10.25%.” Please provide the error analysis and make correction of these values accordingly.

In Table 2, please add the error for the presented values.

Line 250. While discussing the electron structure of their materials, the authors for the first time mention that the structural model was based on the substitution of 50% of Sr atoms with Ba in the SrAl2O4 system. This statement should be thoroughly discussed taking into account the unit cell parameters of the materials prepared at different temperatures, the EDS data, the fact that BaO was found in the material prepared at some particular temperatures.

Fig. 8(d). Please explain how you got this figure. What material did you study?

Line 260. Please explain abbreviations PVA, EA, and PEG.

Line 275. The meaning of the phrase “the more crystalline crystals” is not clear.

Fig. 9. Please discuss the appearance of either two peaks or one peak at 2theta~20 degrees.

Line 284. Please explain the abbreviation PDMS.

The quality of Englih language should be  improved.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors submitted an extensive research on the synthesis, characterization and application of luminescent aluminate powder. The article presents an interesting material with practical application as an anti-couterfeiting ink. Nonetheless, there are multiple flaws, which require major revision before it could be considered for publication in Coatings.

1.       The formula for the studied aluminate is written incorrectly throughout the manuscript. The one time it is written correctly is in the XRD paragraph (Ba0.5Sr0.5Al2O4). In all other places it is written incorrectly as BaSrAl2O4 or Ba0.05Sr0.05Al2O4.

2.       Abbreviations UC and DS should be spelled-out.

3.       There is vastly insufficient background on Ba/Sr aluminates doped with Eu2+ presented in the Introduction. Studies such as: 10.1016/j.optmat.2018.05.076, 10.1364/OME.10.001951, 10.1063/1.4868470 and 10.1016/j.optmat.2012.11.024 should be acknowledged and commented.

4.       There is not indication on how the amounts of the raw materials were calculated in relation to the dopant concentration. Do Eu substitute Ba or Sr or both?

5.       Line 124: The authors write about the large mismatch of IR of Sr and Ba. Is this a mismatch to each other, or the dopant? In the line 132 the above claim is contradicted, by claiming that their IR are similar.

6.       How is the 0.4% of additional phase BaO calculated?

7.       Line 140: The authors suggest that the lattice expansion results from excessive doping. Why would that be, if the dopant has smaller ionic radius?

8.       Fig 1b – the line should be straight to highlight the deviation of the peaks from the initial position.

9.       Fig. 1a – what was the annealing temperature for this concentration series?

10.   Fig. 2f EDS weight percentage should be presented as molar percentage (accounted for molar mass) to demonstrate correct proportions of Ba/Sr/Al/O = 12.5%/12.5%/25%/50%

11.   Values of n should be determined separately for Eg and Eg’ to avoid confusion. The description should be clearer.

12.   The authors do not comment on the origin of two different excitation and emission bands. Are these Eu2+ in different sites? Fig. 6c does no explain the origin of blue and green luminescence. According to the diagram, the transition occurs between the same levels.

13.   Line 193-194: Non-radiative transitions do not enhance emission.

14.   Equation (3) is for energy transfer concentration quenching and should be described as such.

15.   The authors should comment on different ration of 443 nm to 501 nm emission in sample fired at 800 C compared to other temperatures.

16.   In equation (4) I0 does not represent the luminescence intensity, but rather the background constant, which should be subtracted from the data in Fig. 7a,b.

17.   Direct bandgap is not obvious in Fig. 8a,b

18.   The paragraph on electronic structure mentions impact of Eu, which is not studied by DFT and not linked to any modeling result.

19.   The author write, that for modeling 50% of Sr atoms in SrAl2O4 was substituted with Ba. What were the dimensions of the studied structure (how many unit cells did the structure consisted of)? Please include that information in the Experimental.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript can be published.

Minor editing of English language is required.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The authors submitted very well improved revised version of the manuscript. They addressed all my comments and provided satisfactory responses. I have only minor comments that should be addressed before publication:

1. Fig 1a – the authors reply states that the samples were synthesized in “high temperature” but do not specify what was the exact temperature.

2. Fig 6c is still not entirely correct. The excitation at 303 nm and 365 nm corresponds to energies of 4.1 ev and 3.4 eV, respectively, and cannot be the excitation from VB to CB. I believe it is most possibly internal Eu2+ excitation from 4f7 ground state to 4f65d state in two different sites in the structure. Similarly, the emission at 442 nm is not result of transition from the higher 5d level in the same ion as the emission at 500 nm. Most probably there are two distinct sites for Eu2+: in the first the excitation of 303 nm results in emission at 442 nm, and in the second the excitation at 365 nm results in emission at 500 nm. The energy levels of Eu at two sites can be drawn separately.

3. Line 276 I suggest to move the sentence about the impact of Eu doping on the bandgap to the part about absorption.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

All my comments were addressed and I recommend the article to be accepted for publishing.